Resin compositions, each of which contains an olefin resin such as polyethylene or polypropylene and fibers, are widely used as compositions for obtaining moldings reinforced with fibers. In these compositions, sizing compositions are applied to the surfaces of the fibers to improve the compatibility between the olefin resins and the fibers.
A molding of a fiber-reinforced olefin resin composition can be produced by directly mixing and injection-molding an olefin resin and chopped fiber strands; by kneading, extruding and chopping both of them by an extruder or the like into short fiber pellets in advance and then injection-molding the short fiber pellets; or by causing fibers to run parallel each other, impregnating the fibers with a molten resin, pulling and chopping the thus-impregnated fibers into long fiber pellets, and then injection-molding the long fiber pellets.
However, an olefin resin, for its chemical structure, does not contain polar groups on its molecular chain and is low in surface activity. Even when the olefin resin is formed into pellets as described above, the adhesion of the olefin resin to fibers is poor so that the resulting molding would not be provided with strength improved to such sufficient extent as expected. There is another drawback that fibers tend to become loose and to scatter around from the composition so obtained. Resin compositions for fiber-reinforced moldings, which make use of olefin resins as base resins and have been prepared by pultrusion, involve such problems. It has, therefore, been keenly desired to solve these problems.
To solve such problems, it is proposed, for example, in JP 2,941,320 B to use an olefin resin as a fiber-impregnating matrix resin and a modified olefin resin, which has been subjected to a specific modification, in combination such that, when formed into long fiber pellets, synergistic effects are developed with the treatment of the sizing composition for the fibers and the resulting molding is considerably improved in mechanical strength and the like. This patent publication also discloses to use a modified olefin resin as a component of a sizing composition for glass fibers.
The above-described method, however, involves problems. Upon using the modified olefin resin as a component of a sizing composition, it is necessary to provide the modified olefin resin with water dispersibility and then to apply it to fibers. This requires addition of a neutralizing agent or surfactant to the modified olefin resin. Depending on the treatment method for providing the modified olefin resin with water dispersibility or water solubility, the modified olefin resin may not sufficiently adhere to the surfaces of the fibers so that the method is poor in strand integrity (i.e., the processability of forming fibers into a strand or roving). Therefore, fuzz may be formed on pellets and moldings. When molded as short fiber pellets or long fiber pellets, the resulting molding is provided with inferior mechanical strength.
Objects of the present invention is, therefore, to provide a sizing composition for fibers, which makes it possible to firmly bond an olefin resin as a matrix resin and the fibers with each other and hence, to provide a molding having excellent strength without occurrence of fuzzing on the resin pellets or the molding; and also to fibers for olefin resin reinforcement and a production process of an olefin resin composition for fiber-reinforced moldings.